1. Field of the Invention
The present invention relates to improved gypsum board products, and more particularly to gypsum boards having improved properties for use in humid areas.
Gypsum board products comprising a monolithic core of set, interlaced gypsum crystals disposed between fiberous, especially paper, liners are well known in the art. Gypsum board is normally produced by feeding a slurry of calcium sulfate hemihydrate (stucco), water, foam, and any other desired additive such as setting time modifiers, between two sheets of paper and the like fiberous material. After conversion of the stucco into gypsum (calcium sulfate dihydrate) the set but still wet board is conveyed through a drying chamber to remove excess water. Such gypsum core boards, including lath and sheathing boards, have been employed in building construction for a long time in the construction of interior walls and ceilings, and are commonly referred to as gypsum wallboard, plasterboard and so forth. Such boards are presently commercially available in various thicknesses, generally 1/4th inch to 5/8th inch. Paper covered gypsum wallboard currently being marketed in a 1/2 inch thickness generally weighs between 1650 and 2150 lbs. per thousand square feet. At the low end of this range, the gypsum core has a density of about 40-42 lbs. per cubic foot and a compressive strength of about 450 psi.
As is well known in the building industry such gypsum boards may tend to exhibit a visible sagging after installation, particularly in ceiling constructions under humid conditions. This may become quite evident especially in the middle of ceiling mounted board nailed to 16 inch to 24 inch spaced apart supporting members. Certain surface decorations increase the tendency to sag.
2. Description of the Prior Art
Coal fly ash is the portion of very fine ash of furnaces burning powdered coal that passed into the stack and is recovered from the stack gases as a necessary step to decrease pollution of the atmosphere. Coal fly ash is obtained principally by removal from power plant flue gases burning various grades of coal, whether lignite or bituminous, via such means as electrostatic precipitators, venturi scrubbers and the like or collected in a baghouse. Thus, coal fly ash is a synthetic by-product in manufacturing processes and is distinguished not only in method of derivation but also in physical and chemical properties from naturally occuring aluminate or silicate minerals such as perlite ore, vermiculite ore, volcanic ash, colloidal silica and the like.
Recently coal fly ash has been finding increased practical uses in industry either as inert lightweight filler or in combinations with small amounts of gypsum and a lime source to activate its pozzolanic activity. Thus coal fly ash has been suggested along with other fillers for use in compositions such as dense gypsum metal casting molding compositions (U.S. Pat. No. 4,148,760) thermal and sound insulating gypsum construction material (Chem Abstracts 79:3426e and 90:10893h). For example further, fly ash has been added to the Portland cement used in making concrete. Here large quantities of fly ash, in proportions of up to about half of the Portland cement in the cement-aggregate mixture, are added to react with lime generated during the setting of the Portland cement to produce additional cementitious compounds. Utilization of fly ash in this manner often results in substantial cost savings. A saving of over 1 million dollars in the construction costs, for example, of the Hungry Horse dam has been reported. It has been stated that concrete made with Portland cement and coal fly ash has greater strength and less heat of hydration, is more resistant to chemical action by acids, alkalis and sulfates. See Chemical Abstracts 90:75704z; 87:121882u; and 86:160087n.
There have been other attempts to take advantage of the high lime reactivity of coal fly ash. Thus U.S. Pat. No. 2,785,988 discloses a non-hydraulic construction material comprising alpha hemihydrate calcium sulfate, gypsum (dihydrate) and a silica highly reactive to lime such as coal fly ash, to obtain accelerated sets and high compressive strengths in the admixtures.
Naturally occuring silicate minerals such as vermiculite ore and perlite ore have been incorporated in the manufacture of gypsum boards generally to add higher fire resistance properties in the case of the unexpanded natural ores or lighter weight and lower density in the case of the fluffy expanded particles. See for example U.S. Pat. Nos. 2,009,146 and 3,376,147. Natural mineral silicates are a common constituent in a great many materials since silicon oxide is so abundantly found in inorganic matter. Thus, certain clays, volcanic glasses, lava fragments and the like that are glassy in character and contain silica have been used as aggregates in gypsum board manufacture. See U.S. Pat. No. 1,574,252.
Concerning another additive unrelated other than that it is said to be high in silicate content and a by-product of certain manufacturing processes, U.S. Pat. No. 4,087,285 discloses a method for strengthening glasses and other vitreous products by adding certain condensed flue gases or "filter dusts" obtained from specific manufacturing processes as a flux. The filter dusts disclosed here appear to have as their characteristic feature a uniform minute particle size with enormous specific surface in excess of 20 m.sup.2 /g, or 200,000 cm.sup.2 /g, and a very high silicate content. The primary example of such waste dust appears to be the condensed flue gases in the manufacture of ferro-silicon, as a type of synthetic "fumed silica". Another principal type disclosed appears to be the fine powdery waste from perlite expansion said to contain 70-75% SiO.sub.2 and 12-15% Al.sub.2 O.sub.3 which again would have the averred characteristic uniform minute particle size and enormous specific surface. Such materials are alleged in this patent to also be usable in contributing increased mechanical strength in the manufacture of boards such as gypsum board by some unspecified action mechanism.